Add Musig2 Support for FFM

[liamgilligan]

Basic musig signing capabilities implemented and tested using the BIP-327 test vectors.

Things needed before merge:

• Add support for tweaks
• Remove test helper functions from SecpForeign
• Improve test coverage
• Decide on API
• Break into more atomic commits
• Add more thorough documentation

[liamgilligan]

Added in all the BIP-327 test vectors. It is certainly possible there is a much better way of parsing the .jsons than what I am currently doing. My main priority for this most recent push was simply having the tests run all the test vectors. I may clean them up soon.

[liamgilligan]

I was able to use JsonNodes to significantly cleanup the code, removing around ~10 unnecessary records.

[msgilligan]

The failing tests are most likely because the Ubuntu 24.04 GItHub Actions runners are using libsecp256k1 v0.2.0.

4a91bb3 updates to Ubuntu 26.04 (which has libsecp v0.7.0) and should fix the broken tests.

[liamgilligan]

Began cleaning up API by creating interfaces and impls for new return types. There needs to be someway to handle libsec's opaque data structures (namely musig_keyagg_cache and secp256k1_musig_secnonce) in a clean way -- if we must avoid recreating these opaque structures as they are subject to change (namely the magic byte prefix, IIRC) and will introduce difficulties in making the API implementation agnostic.

[msgilligan]

First glance at rebase looks good! Thanks!